The present invention relates generally to pneumatically reciprocable burrowing tools, having an adjustable capability for forwardly or rearwardly burrowing through soil. More particularly, the invention relates to an improved burrowing tool design which permits an overall reduction in tool size without loss of the reversible drive qualities of the burrowing tool.
Pneumatically operated burrowing tools are well known in the art, usually operating on the principle of pneumatically reciprocating a piston hammer against an anvil to create a mechanical shock impact, the direction of such impact causing perceptible movement of the tool through soil. When the mechanical shock impact is driven in a direction toward the front of the tool the tool movement is forwardly into the soil, and when the mechanical shock impact is delivered in a direction towards the rear of the tool the tool movement through the soil is rearwardly. Such tools have been developed wherein the hammer drive mechanism may be selectively adjusted to cause the tool to move in either direction, thereby permitting the use of the tool for burrowing a tunnel through the soil in one direction, and reversing the action of the tool so as to permit the tool to back out of the tunnel to the point of beginning. The prior art shows various mechanical adaptations for accomplishing this reversing function, usually by controlling the stroke of the piston hammer so as to permit a shock impact at either a forwardly positioned anvil or a rearwardly positioned anvil inside of the tool.
Since such tools are pneumatically operated, it is necessary that they be coupled to a pressurized air hose, and a source of pressurized air be connected into the hose for providing the necessary driving energy. The valving mechanism inside of the tool usually permits the pressurized air in the hose to be directed to drive the piston hammer in a first direction, exhausting air through the rear end of the tool, and also to passages for driving the piston hammer in a reverse direction. Therefore, during each complete stroke of the piston hammer there is a force stroke wherein pressurized air moves the piston against an anvil and a return stroke wherein pressurized air returns the piston to an initial position. During each stroke air is exhausted from the rear of the device. The exhaust air is typically directed through holes at the rear of the device into the soil or into the tunnel which has been created by operation of the device.
The valving mechanism inside the tool which typically regulates the forward and reverse strokes of the piston may be adjusted in some pneumatic burrowing tools to permit reversible operation. In such tools, the valving mechanism may be adjusted to a first position permitting reciprocable motion of the piston hammer over a predetermined stroke, so as to cause the hammer to impact against a forward anvil. The valving mechanism may be repositioned to a second position wherein the piston hammer is reciprocable over a predetermined stroke, such that the piston hammer impacts against a rearward anvil. In such tools, the impact of the hammer against the forward anvil causes forward motion of the burrowing tool through soil, and the impact of the hammer against the rearward anvil causes rearward motion of the tool through soil. One of the design approaches utilized for enabling the selective positioning of the valving mechanism is an interior threaded portion in the tool which engages a threadable sleeve coupled to a spool valve, and also engages an air hose connected to the tool. This mechanism permits the valving mechanism to be threadably shifted along the tool axis by twisting the air hose a predetermined number of turns, which can be accomplished by an operator located at some distance from the burrowing tool, but having access to the air hose for purposes of twisting it.
In all burrowing tools known in the prior art the pressurized air applied to the tool for purposes of impacting the tool in either a forward or reverse direction is exhausted through the rear of the tool into the soil. The exhaust function is controlled by the same valving mechanism as controls the driving function, internal passages in the tool permit the exhaust air to escape through the rear end cap of the tool. Since these exhaust passages are provided in the body of the tool itself, the overall size of the tool must be made large enough to accommodate these passages, as well as to accommodate the driving piston and valving mechanism. At the same time, the tool size must be made large enough to withstand the impact forces which are developed inside the tool, necessarily compensating for the loss of material caused by forming the exhaust passages.